Recently, a nonaqueous electrolyte battery such as a lithium-ion secondary battery has been developed as a battery having a high energy density. The nonaqueous electrolyte battery is expected to be used as a power source for hybrid vehicles or electric cars. Further, it is expected to be used as an uninterruptible power supply for base stations for portable telephone, and the like. For this, the nonaqueous electrolyte battery is desired to have other performances such as rapid charge/discharge performances and long-term reliability. For example, a nonaqueous electrolyte battery enabling rapid charge/discharge not only remarkably shortens the charging time but also makes it possible to improve performances of the motive force of a hybrid vehicle and to efficiently recover the regenerative energy of them.
In order to enable rapid charge/discharge, it is necessary that electrons and lithium ions can migrate rapidly between the positive electrode and the negative electrode. When a battery using a carbon based material in the negative electrode repeats rapid charge/discharge, dendrite precipitation of metal lithium is occurred on the electrode. Dendrite causes internal short circuits, which can lead heat generation and fires.
In light of this, a battery using a metal composite oxide as a negative electrode active material in place of a carbonaceous material has been developed. Particularly, in a battery using a titanium oxide as the negative electrode active material, rapid charge/discharge can be performed stably. Such a battery also has a longer life than those using a carbonaceous material.
However, the titanium oxide has a higher (nobler) potential than the carbonaceous material relative to metal lithium. Further, the titanium oxide has a lower capacity per weight. Thus, a battery formed by using the titanium oxide has a problem such that the energy density is low.
The potential of the electrode using the titanium oxide is about 1.5 V based on metal lithium and is higher (nobler) than that of the negative electrode using the carbonaceous material. The potential of the titanium oxide is due to the redox reaction between Ti3+ and Ti4+ when lithium is electrochemically inserted and released. Therefore, it is limited electrochemically. Further, there is the fact that rapid charge/discharge of lithium ion can be stably performed at an electrode potential as high as about 1.5 V. Therefore, it is substantially difficult to drop the potential of the electrode to improve energy density.
As to the capacity of the battery per unit weight, the theoretical capacity of a lithium-titanium composite oxide such as Li4Ti5O12 is about 175 mAh/g. On the other hand, the theoretical capacity of a general graphite type electrode material is 372 mAh/g. Therefore, the capacity density of the titanium oxide is significantly lower than that of the carbon type material. This is due to a reduction in substantial capacity because there are only a small number of lithium-adsorbing sites in the crystal structure and lithium tends to be stabilized in the structure.
In view of such circumstances, a new electrode material containing Ti and Nb has been examined. Such a material is expected to have high charge/discharge capacity. Particularly, the theoretical capacity of a composite oxide represented by TiNb2O7 exceeds 300 mAh/g. However, there is a problem such that, in such an oxide which reacts at a high potential like about 1.5 V vs Li/Li+, any surface film is hard to be formed, and thus decomposition of an electrolyte solution on the electrode surface (that is, a side reaction) is easily continued.